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Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Private Education Loans allowance for losses | $ (1,413,621) | $ (1,269,652) | $ (1,339,772) | $ (1,416,048) | $ (1,364,716) | $ (1,357,075)
Private Education Loans, net | $ 20,459,933 |  | $ 20,306,357 |  |  |
Percentage of Private Education Loans in repayment, Year 1 | 54.80% |  | 55.60% |  |  |
Percentage of Private Education Loans in repayment, Year 2 | 58.50% |  | 65.30% |  |  |
Percentage of Private Education Loans in repayment, Year 3 | 69.70% |  | 75.20% |  |  |
Percentage of Private Education Loans in repayment, Year 4 | 77.30% |  | 81.10% |  |  |
Percentage of Private Education Loans in repayment, Year 5 | 82.20% |  | 85.00% |  |  |
Percentage of Private Education Loans in repayment, After Year 5 | 89.00% |  | 89.30% |  |  |
Percentage of Private Education Loans in repayment | 70.50% |  | 73.30% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1 | 0.30% |  | 1.00% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2 | 1.70% |  | 2.10% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3 | 3.70% |  | 3.60% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4 | 4.30% |  | 4.10% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5 | 4.20% |  | 4.50% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5 | 5.90% |  | 6.80% |  |  |
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment | 3.60% |  | 3.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1 | 0.20% |  | 0.80% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2 | 1.90% |  | 2.70% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3 | 2.90% |  | 2.60% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4 | 2.40% |  | 2.30% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5 | 2.30% |  | 2.20% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5 | 2.00% |  | 1.90% |  |  |
Loans in forbearance as a percentage of loans in repayment and forbearance | 1.90% |  | 2.10% |  |  |
Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in Repayment, Year 1 | $ 2,271,922 |  | $ 2,494,814 |  |  |
Total Private Education Loans in Repayment, Year 2 | 3,136,672 |  | 3,323,327 |  |  |
Total Private Education Loans in Repayment, Year 3 | 2,250,064 |  | 2,209,734 |  |  |
Total Private Education Loans in Repayment, Year 4 | 1,634,932 |  | 1,475,858 |  |  |
Total Private Education Loans in Repayment, Year 5 | 1,165,916 |  | 1,386,889 |  |  |
Total Private Education Loans in Repayment, After Year 5 | 4,900,749 |  | 4,519,192 |  |  |
Total Private Education Loans in repayment | 15,360,255 |  | 15,409,814 |  |  |
Total Private Education Loans, gross, Year 1 | 4,145,008 |  | 4,490,119 |  |  |
Total Private Education Loans, gross, Year 2 | 5,363,248 |  | 5,088,739 |  |  |
Total Private Education Loans, gross, Year 3 | 3,229,776 |  | 2,937,422 |  |  |
Total Private Education Loans, gross, Year 4 | 2,113,786 |  | 1,819,089 |  |  |
Total Private Education Loans, gross, Year 5 | 1,419,153 |  | 1,632,300 |  |  |
Total Private Education Loans, gross, After Year 5 | 5,506,495 |  | 5,058,175 |  |  |
Total Private Education Loans, gross | 21,777,466 |  | 21,025,844 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1 | 37,794 |  | 35,616 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2 | 26,654 |  | 18,556 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3 | 11,375 |  | 9,465 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4 | 6,512 |  | 5,809 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5 | 4,325 |  | 3,556 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5 | 9,428 |  | 8,552 |  |  |
Private Education Loans deferred origination costs and unamortized premium/(discount) | 96,088 |  | 81,554 |  |  |
Total Private Education Loans, Year 1 | 4,182,802 |  | 4,525,735 |  |  |
Total Private Education Loans, Year 2 | 5,389,902 |  | 5,107,295 |  |  |
Total Private Education Loans, Year 3 | 3,241,151 |  | 2,946,887 |  |  |
Total Private Education Loans, Year 4 | 2,120,298 |  | 1,824,898 |  |  |
Total Private Education Loans, Year 5 | 1,423,478 |  | 1,635,856 |  |  |
Total Private Education Loans, After Year 5 | 5,515,923 |  | 5,066,727 |  |  |
Total Private Education Loans | 21,873,554 |  | 21,107,398 |  |  |
Private Education Loans allowance for losses, Year 1 | (212,169) |  | (269,642) |  |  |
Private Education Loans allowance for losses, Year 2 | (313,135) |  | (335,090) |  |  |
Private Education Loans allowance for losses, Year 3 | (235,225) |  | (194,104) |  |  |
Private Education Loans allowance for losses, Year 4 | (155,737) |  | (118,755) |  |  |
Private Education Loans allowance for losses, Year 5 | (95,729) |  | (100,111) |  |  |
Private Education Loans allowance for losses, After Year 5 | (401,626) |  | (317,403) |  |  |
Private Education Loans allowance for losses | (1,413,621) |  | (1,335,105) |  |  |
Private Education Loans, net, Year 1 | 3,970,633 |  | 4,256,093 |  |  |
Private Education Loans, net, Year 2 | 5,076,767 |  | 4,772,205 |  |  |
Private Education Loans, net, Year 3 | 3,005,926 |  | 2,752,783 |  |  |
Private Education Loans, net, Year 4 | 1,964,561 |  | 1,706,143 |  |  |
Private Education Loans, net, Year 5 | 1,327,749 |  | 1,535,745 |  |  |
Private Education Loans, net, After Year 5 | 5,114,297 |  | 4,749,324 |  |  |
Private Education Loans, net | 20,459,933 |  | 19,772,293 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent, current |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 2,264,429 |  | 2,469,817 |  |  |
Year 2 | 3,081,961 |  | 3,254,534 |  |  |
Year 3 | 2,166,187 |  | 2,131,040 |  |  |
Year 4 | 1,564,016 |  | 1,416,069 |  |  |
Year 5 | 1,116,464 |  | 1,323,825 |  |  |
After Year 5 | 4,613,926 |  | 4,213,986 |  |  |
Loan type | 14,806,983 |  | 14,809,271 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 5,536 |  | 17,599 |  |  |
Year 2 | 31,829 |  | 34,627 |  |  |
Year 3 | 41,997 |  | 37,147 |  |  |
Year 4 | 34,974 |  | 28,020 |  |  |
Year 5 | 23,902 |  | 31,432 |  |  |
After Year 5 | 147,233 |  | 149,926 |  |  |
Loan type | 285,471 |  | 298,751 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 1,314 |  | 5,720 |  |  |
Year 2 | 15,492 |  | 17,227 |  |  |
Year 3 | 24,717 |  | 20,077 |  |  |
Year 4 | 20,840 |  | 16,614 |  |  |
Year 5 | 14,467 |  | 15,482 |  |  |
After Year 5 | 72,268 |  | 75,897 |  |  |
Loan type | 149,098 |  | 151,017 |  |  |
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 643 |  | 1,678 |  |  |
Year 2 | 7,390 |  | 16,939 |  |  |
Year 3 | 17,163 |  | 21,470 |  |  |
Year 4 | 15,102 |  | 15,155 |  |  |
Year 5 | 11,083 |  | 16,150 |  |  |
After Year 5 | 67,322 |  | 79,383 |  |  |
Loan type | 118,703 |  | 150,775 |  |  |
Loans In-school/grace/deferment | Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Year 1 | 1,868,692 |  | 1,976,040 |  |  |
Year 2 | 2,166,252 |  | 1,672,333 |  |  |
Year 3 | 912,688 |  | 669,250 |  |  |
Year 4 | 438,760 |  | 307,781 |  |  |
Year 5 | 225,235 |  | 213,593 |  |  |
After Year 5 | 504,170 |  | 452,994 |  |  |
Loan type | 6,115,797 |  | 5,291,991 |  |  |
Loans In forbearance | Consumer Portfolio Segment | Student Loan |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loans in forbearance, Year 1 | 4,394 |  | 19,265 |  |  |
Total Private Education Loans in forbearance, Year 2 | 60,324 |  | 93,079 |  |  |
Total Private Education Loans in forbearance, Year 3 | 67,024 |  | 58,438 |  |  |
Total Private Education Loans in forbearance, Year 4 | 40,094 |  | 35,450 |  |  |
Total Private Education Loans in forbearance, Year 5 | 28,002 |  | 31,818 |  |  |
Total Private Education Loans in forbearance, After Year 5 | 101,576 |  | 85,989 |  |  |
Total Private Education Loans in forbearance | $ 301,414 |  | $ 324,039 |  |  |

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